In present day oil well drilling operations it is becoming increasingly important to have the capability of selectively controlling the directional course of the drill bit. Such controlled directional drilling particularly important in any offshore operation where a number of wells are to be drilled from a central drilling platform or vessel so as to individually reach selected target areas respectively situated at different depths, azimuthal orientations and horizontal displacements from the drilling platform. Moreover, in any offshore or inland drilling operation, there are many situations where the drill bit must be deliberately diverted laterally to complete the drilling of the borehole.
Those skilled in the art will, of course, appreciate that many types of directional drilling tools have been proposed in the past. For instance, one of the best known tools in use today is a so-called "whipstock tool" which is cooperatively arranged for drilling a reduced diameter pilot hole in a desired lateral direction and inclination from the original borehole course. The use of such whipstock tools necessitates removal of the drill string to install a special whipstock guide and a reduced size drill bit in the borehole. Special measuring devices are then employed to position the whipstock as required for drilling the pilot hole in a given direction. The guide and its associated bit are subsequently removed and the drill string and original bit are returned to the borehole to resume drilling of the borehole along the deviated pilot hole. It is, therefore, apparent that such whipstock operations are too time-consuming and unduly expensive to be feasible except in extreme situations.
Perhaps the most common directional drilling technique in use today utilizes specially arranged drilling apparatus commonly called a "big eye" drill bit which has one of its several fluid nozzles enlarged and arranged to discharge a jet of the drilling mud in a selected lateral direction. To utilize these jet-deflection bits, rotation of the drill string is temporarily discontinued. By utilizing a typical orienting tool the drill string is manipulated so as to position the big eye bit with its enlarged nozzle facing in the direction in which the borehole is to be subsequently deviated. The mud pumps of the drilling rig are then operated so that a concentrated jet of the circulating drilling mud is forcibly discharged against the adjacent borehole wall surface so as to progressively erode away or carve out a cavity on that side of the borehole. Once it is believed that an adequate cavity has been carved out, the drilling operation is resumed with the expectation that the drill bit will be diverted into the eroded cavity and thereby initiate the desired deviation of the borehole. Typical tools of this nature are described, for example, in U.S. Pat. Nos. 3,360,057, 3,365,007, 3,488,765 and 3,599,733.
Those skilled in the art will recognize, of course, that such prior art jet deflection tools require many time-consuming directional measurements to correctly position the drill bit. It should also be recognized that while cutting a cavity with such prior art tools, the rate of penetration will be significantly decreased since the drill string can not be rotated during such prolonged operations. Thus, these prior art tools are not particularly efficient for deviating boreholes at extreme depths or those situated in hard earth formations. Moreover, since the drill string must be maintained stationary during the jetting operation, in some instances the drill string may possibly be subjected to so-called "differential sticking" at one or more locations in the borehole. Accordingly, heretofore other types of directional drilling tools have been proposed for redirecting the borehole without having to discontinue rotation of the drill string. One of the earlier tools of this nature is found in U.S. Pat. No. 2,075,064. In that tool, a valve is cooperatively arranged in a conventional drill bit and is controlled by a pendulum member with an eccentrically located center of gravity to equalize the discharge rate of drilling fluid from each of the bit nozzles to ensure that the drill bit will continue to follow a previously drilled pilot hole. Those skilled in the art will, of course, recognize that this particular apparatus is itself incapable of initiating a change in direction of a borehole. U.S. Pat. Nos. 3,593,810 and 4,307,786 respectively depict two directional drilling tools which are each selectively energized as rotation of the drill string carries a wall contacting member into momentary contact with the lower wall of an inclined borehole interval. The tool described in the first of these two patents is cooperatively arranged so that as the drill string is rotated, the periodic contact of the actuating member with the borehole wall is effective to selectively extend a laterally movable guiding member on the tool and thereby continuously urge the drill bit in a given lateral direction. The tool described in the second of these two patents is provided with a source of pressured fluid. In response to the periodic engagement of the wall contacting actuator with the lower wall of the borehole, the pressured fluid is repetitively emitted from a selected nozzle in a conventional drill bit so as to continuously direct the pressured fluid against only a selected circumferential portion of the borehole. Thus, continued operation of this prior art tool will be effective for progressively diverting the drill bit toward that portion of the borehole wall. Those skilled in the art will recognize, of course, that these two prior art tools are wholly dependent upon their respective actuating members being able to contact the borehole wall above the drill bit. Thus, should there be portions of the borehole wall which are so washed out that they cannot be contacted when these actuating members are fully extended, these particular tools will be incapable of operating properly in that borehole interval.